The present invention relates generally to high silicon corrosion resistant alloys and, more particularly, to nickel-based alloys which are used in corrosive environments, for example, environments containing hot mineral acids.
Applicant is aware of the following U.S. patents, the disclosures of which are incorporated by reference herein:
Equipment used in highly corrosive environments typically is constructed of metal alloys designed to offer greater corrosion resistance, such as stainless steel or other high alloys. These alloys are necessary to withstand the extremely corrosive effects of chemicals such as hot concentrated sulfuric acid and the like. High silicon alloys such as Hastelloy-D(trademark) (Ni-10Si-3Cu) and Duriron(trademark) (Fe-15Si-3.5Mo) are commonly used in the chemical processing industry for these environments. These alloys provide good high temperature corrosion resistance in these environments within limitations. However, both alloys are susceptible to brittle failure due to poor ductility, for example, less than 1% elongation on a tensile test. Equipment manufactured from these alloys is susceptible to failure when subjected to either mechanical or thermal shock loads and the very low ductility limits the use of these alloys in fabrication of some parts.
A typical application requiring use of high alloy material is the recovery of spent sulfuric acid. The production of titanium dioxide via the sulfate process results in a waste stream of 20% sulfuric acid. Historically, this waste stream was disposed of by a variety of methods, such as, deep well injection or neutralization followed by landfill. Environmental concerns have eliminated these disposal methods. A common method of recovering this waste stream is to increase the acid concentration to 93%, for example, using a forced circulation evaporator. The concentrated acid may then be reused in the production of titanium dioxide. The acid concentration is normally carried out in multiple stages and the acid temperature is kept as high as possible to minimize vacuum requirements. Stainless steel or other high alloy materials offer acceptable corrosion resistance in the 20 to 60% and 80 to 93% concentration range at temperatures approaching the boiling point. However, metallic components handling acid in the 60 to 80% concentration range at high temperature must be handled with Ni-9Si-3Cu (Hastelloy D(trademark)) alloy or Fe-15.5Si-3.5Mo alloy (Duriron(trademark)), based on acceptable corrosion resistance.
The alloy of this invention offers significantly improved life for fluid handling equipment in this environment and in similar corrosive environments. In particular, the greatly improved ductility increases the durability of parts formed from the instant alloy.
Nickel and iron based alloys have long been known to be highly resistant to sulfuric acid. Alloys with as much as 14.5% Si have been used for a broad range of concentrations and up to the boiling point of sulfuric acid. This high resistance comes at the cost of low tensile strengths and extreme brittleness. A conventional alloy used in concentrated sulfuric acid is Hastelloy D(trademark), which is a nickel based alloy containing silicon and copper. Although Hastelloy D(trademark) shows excellent corrosion resistance in concentrated sulfuric acid, this alloy is hard and brittle.
The instant invention overcomes this brittleness in conventional nickel-based high silicon alloys and yet retains, and may improve, the corrosion resistance.
In accordance with the present invention, there is provided a nickel-based alloy having high corrosion resistance to severe sulfuric acid environments. The alloy may have a nickel content of between about 82 to 86%, a silicon content of between about 9 to 11% by weight, preferably between about 9.8 to 10.6%, a niobium content of between about 3 to 6% by weight, preferably between about 3.3 to 5.5% and more preferably about 3.3 to 4.4%, a boron content of between about 0-0.11%, a cerium content of between about 0% to 0.10%, and the balance comprising misch-metal (50% cesium, 20% lanthanum; 20% neodymium). These percentages are by weight. The alloy of the present invention has a high resistance to corrosion while maintaining sufficient ductility for machining. A ductility of greater than 1% elongation to fracture can be achieved. This ductility is effective to be cast and machined. The alloy is resistant to mechanical and thermal shock. The alloy can be used in rotating machinery, such as pump parts, which require mechanical strength.
It is an object of the present invention to provide an alloy for use in highly corrosive environments.
It is an object of the present invention to provide an alloy which is resistant to corrosion in a concentrated sulfuric acid environment.
It is an object of the present invention to provide an alloy which is resistant to corrosion and has sufficient ductility to be fabricated and survive in service.
It is an object of this invention to provide a high silicon nickel based alloy having high corrosion resistance in concentrated sulfuric acid environments.
It is an object of this invention to provide a high silicon nickel based alloy having improved ductility.
It is a further object of this invention to provide a high silicon nickel based alloy which is substantially free of elements which reduce alloy ductility, while providing an alloy having high corrosion resistance in hot mineral acids, such as hot concentrated sulfuric acid.
It is a further object of this invention to provide a high silicon nickel based alloy which is castable, machinable and weldable.
These and other objects of the invention will be apparent from the following Description of the Preferred Embodiments.